1. Field of the Invention
The present invention relates to an image-reading apparatus, and particularly to an illuminating optical system thereof.
2. Description of the Related Art
In the image sensors of a facsimile, copying machine, scanner, etc. which read reflected light from documents to be read, light-emitting diodes (or LED) are widely used as illuminating optical systems.
The invention disclosed in JP-A-2001-136341 relates to the basic structure of a color-sequentially reading type contact image sensor, and the invention provides an image sensor which shows uniform output levels of red, green and blue and has a high sensitivity. The image-reading apparatus disclosed in JP-A-11-215301 is intended to improve the quality of read images as follows: a simple means is used to efficiently guide light emitted from a light source to an image-reading region, and thereby, the illuminance on the image-reading region is enhanced to improve the quality of the read image. However, this apparatus has a defect in that, since the surface of the hollow light-guiding body is colored white, most of light is diffused, and therefore, it is difficult to efficiently guide light to the surface of a document.
The invention disclosed in JP-A-11-55456 is intended to reduce the production cost of the image-reading apparatus by facilitating the assembly of predetermined components such as a light source, etc. and the electric wiring, which compose the image-reading apparatus, without any disadvantage in the image-reading functions. However, this image-reading apparatus has a problem in that the number of LEDs to be increased in order to enhance the intensity of illumination is limited, because light from LEDs are led to the light-guiding body, only from the side wall or the center of the base of the light-guiding body: i.e., the opening of the light-guiding body for introducing LED is small.
The invention disclosed in JP-A-2000-349957 provides a color image sensor capable of efficiently and uniformly guiding light from a light source to a reading region, and an image-reading apparatus comprising the same. However, the angle of outgoing light becomes larger since light is guided by a curved prism, and thus, the illuminated area of the surface of a document becomes larger. As a result, the efficiency of utilizing illuminating light is poor in case of linear reading.
The invention disclosed in JP-A-2001-77975 makes it possible to efficiently and correctly guide light emitted from the light source of an image-reading apparatus, to a desired linear objective region to be read. However, this apparatus has problems in that the shapes of lenses arrayed are complicated, and also that high accuracy is demanded for production of the apparatus.
The illuminating optical systems of the existing image-reading apparatuses which comprise the inventions disclosed in the above publications have problems in that the illumination angles are polarized because light fluxes directly illuminate the surfaces of documents from the light-guiding means, and therefore that the wrinkles on the surfaces of the documents cast their shadows, in other words, undesirably, the reading optical systems read such shadows.